This issue affects the version of the ruby package, as shipped with Red Hat Enterprise Linux 6.
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This issue affects the versions of the ruby package, as shipped with Fedora release of 14 and 15. Please schedule an update.

I've not heard anything back from MITRE about the difference, but I've looked at the CVE entry for CVE-2011-2686 and it reads:
Ruby before 1.8.7-p352 does not reset the random seed upon forking, which makes it easier for context-dependent attackers to predict the values of random numbers by leveraging knowledge of the number sequence obtained in a different child process, a related issue to CVE-2003-0900. NOTE: this issue exists because of a regression during Ruby 1.8.6 development.
Which has me a bit more confused. By the sounds of the description, CVE-2011-2686 was fixed in 1.8.6, but regressed which caused the flaw in 1.8.7 (I have no idea if this is the case).
I suspect we should use CVE-2011-2686 to describe any fixes in 1.8.6 and CVE-2011-3009 to describe any fixes in 1.8.7.